Patients with kidney failure or partial kidney failure typically undergo hemodialysis treatment, often at a hemodialysis treatment center or clinic. When healthy, kidneys maintain the body's internal equilibrium of water and minerals (e.g., sodium, potassium, chloride, calcium, phosphorous, magnesium, and sulfate). The kidneys also function as part of the endocrine system to produce the hormone erythropoietin as well as other hormones. Hemodialysis is an imperfect treatment to replace kidney function, in part, because it does not correct the endocrine functions of the kidney.
In hemodialysis, blood is taken from a patient through an intake needle (or catheter) which draws blood from an artery located in a specific accepted access location (arm, thigh, subclavian, etc.). The drawn blood is pumped through extracorporeal tubing via a peristaltic pump, and then through a special filter termed a “dialyzer.” The dialyzer is intended to remove unwanted toxins such as blood urea, nitrogen, potassium, and excess water from the blood. As the blood passes through the dialyzer, it travels in straw-like tubes which serve as semi-permeable membrane passageways for the uncleaned blood. Fresh dialysate liquid, which is a solution of chemicals and water, flows through the dialyzer in the direction opposite the blood flow. As the dialysate flows through the dialyzer, it surrounds the straw-like membranes in the dialyzer. These membranes feature small holes which are large enough to pass liquid and liquid based impurities—but are not large enough to pass red blood cells. The fresh dialysate collects excess impurities passing through the straw-like tubes by diffusion, and also collects excess water through an ultrafiltration process due to a pressure drop across the membranes. During this process, the red cell volume is preserved inside the straw-like tubes and recirculated back into the body. The used dialysate exits the dialyzer with the excess fluids and toxins via an output tube, thus cleansing the blood and red cell volume flowing through the dialyzer. The dialyzed blood then flows out of the dialyzer via tubing and a needle (or catheter) back into the patient. Sometimes, a heparin drip or pump is provided along the extracorporeal blood flow loop in order to prevent red cell clotting during the hemodialysis process. Several liters of excess fluid can be removed during a typical multi-hour treatment session. In the U.S., a chronic patient will normally undergo hemodialysis treatment in a dialysis center three times per week, either on Monday-Wednesday-Friday schedule or a Tuesday-Thursday-Saturday schedule. These in-center treatments are typically completed over 3 to 4 hours with blood flow rates typically above 300 ml/minute. In other countries, the flow rates and time for treatment are lower and longer, respectively.
Hemodialysis has an acute impact on the fluid balance of the body due in part to the rapid change in circulating blood volume. When the dialysis fluid removal rate is more rapid than the plasma refilling rate of the stored plasma held by the internal tissue of the body, intravascular blood volume decreases. The resulting imbalance has been linked to complications similar to conventional blood loss such as hypotension, loss of consciousness, headaches, vomiting, dizziness and cramps experienced by the patient, both during and after dialysis treatments. Continuous quantitative measurement of parameters relating to the processing of the blood volume (in real-time) during hemodialysis can reduce the chance of dialysis-induced hypotension, and otherwise optimize dialysis therapy regimens by controlling fluid balance and aiding in achieving the target dry weight for the patient.
The effectiveness of the dialyzer in filtering the patient's blood directly affects the patient outcome. Dialyzers are rated by how effective they are at removing the toxins in the patient's blood. This rating is termed a dialyzer's “Clearance,” which is a rate at which blood passes through the dialyzer, expressed as a volume of blood passing through the dialyzer per unit of time.